This invention relates to drilling and cementing boreholes.
The drilling of boreholes is generally carried out using a rotary drilling process. The rotary drilling of a borehole is accomplished by rotating a drill string having a drill pipe and a drill bit at its lower end. Weight is applied to the drill bit while rotating to create a borehole into the earth. The drill string is hollow and sections are added to the drill string to increase its length as the borehole is deepened. This rotary drilling process creates significant amounts of friction which produces heat along with fragments of the strata being penetrated. The fragments of the strata must be removed from the borehole and the drill bit must be cooled to extend its useful life. Both of these necessities are accomplished by the circulation of a fluid down through the drill string and up to the surface through the annulus between the drill string and the wall of the borehole.
Once the borehole has been drilled to the desired depth, it may be desirable to isolate the separate areas, zones or formations transversed by the borehole. For extraction of fluids from formations, and for protection against borehole collapse, one or more conduits (casings) must be inserted into the borehole extending from the surface downward. Thereafter, liners may be hung inside the casings.
At this point it becomes necessary to fill the annulus between the casing and the borehole wall or between the liner and casing with a cementitious material which will seal the annulus to inhibit communication between various formations penetrated by the wellbore and which will provide structural support for the casing or liner. This is commonly referred to as primary cementing.
Generally, the borehole, into which the casing or liner to be cemented is installed, is filled with drilling mud. Therein lie several problems. Conventional Portland cement and conventional drilling muds are incompatible. Thus, as the cement is forced down the casing or liner and up into the annulus it is commingled with the drilling mud at any interface between the mud and the cement. The resulting mixture generally thickens or becomes a gel and does not set up into a strong cement. In addition, the gel strength and viscosity become uncontrollable and the mixture may either become too viscous to pump or may get thinner. If the mixture gets thinner it can allow solids to settle downward in the annulus where they may bridge and restrict the passage of the cement slurry. In either event, the situation is unsatisfactory.
The industry has developed a complex system to attempt to circumvent this problem. A device generally known as a landing collar is attached to the bottom of the casing or liner being cemented. The landing collar has an annular shoulder projecting inwardly. A first wiper plug with a diaphragm which can be ruptured is introduced into the casing or liner followed by a spacer fluid, the cementitious slurry and finally, a solid wiper plug. Displacement fluid then pushes the solid wiper plug downward thus displacing the spacer fluid and the cementitious slurry through the ruptured first wiper plug, out of the casing or liner and into an area at the bottom thereof where it changes direction and flows upwardly into the annulus. When the second wiper plug reaches the area of the landing collar it is stopped by the first wiper plug which is resting on the shoulder.
The spacer fluid, however, can create its own set of problems because it mixes somewhat with both the drilling mud ahead of it and the cementitious slurry behind it. Usually, the most damaging is the contamination of the drilling mud. The spacer fluid remains a fluid in admixture with the drilling fluid and thus cannot be easily separated therefrom. In many instances this necessitates the disposing of the thus-contaminated drilling fluid. This is an economic problem with any drilling fluid but with more expensive, less environmentally friendly oil based fluids, it is a major obstacle to successful operation. In addition, the mixture of spacer fluid and drilling fluid usually results in a more viscous material thus exacerbating the pumping problems. This can even result in a total failure of the cement job because a good seal at the top of the liner may not be obtained and this is where any cement weakened by drilling fluid contamination most likely will be.
It has been found by Hale and Cowan, U.S. Pat. No. 5,058,679 (Oct. 22, 1991) that blast furnace slag is compatible with drilling fluids, and that excellent cementitious compositions can be produced by combining blast furnace slag and drilling fluids. Even blast furnace slag, however, can contaminate oil based drilling muds when the two are mixed. Parceveaux et al., U.S. Pat. No. 5,101,902 (Apr. 7, 1992) broadly discloses spacers between mud and cement and discloses adjusting the density with weighting agents including barite and fillers.
Oil based drilling fluids are disclosed in Shell Internationale patent GB 2,223,255A (Apr. 4, 1990). Cowan, U.S. Pat. No. 5,016,711 (May 21, 1991) broadly discloses improved cement adhesion through the use of a surfactant.